i) Field of the Invention
This invention relates to crystalline sulphated cellulose II and its production especially from the sulphuric acid hydrolysis of cellulose, the invention also relates to the concurrent production of crystalline, sulphated cellulose I and sulphated cellulose II materials.
ii) Description of the Prior Art
Cellulose is the most abundant biopolymer on earth. It is the main component of higher plant cell walls, and it is also formed by some algae, fungi, bacteria, and a group of invertebrate marine animals, the tunicates [See reference by French et al.]. Native cellulose or cellulose from pulping of lignocellulosic materials is fibrous and consists of crystalline and amorphous domains of 1,4-linked β-D-glucose. The crystal structures (polymorphs) of cellulose vary depending on the source of cellulose and the method for its isolation. Because of the existence of various polymorphs, native cellulose has been named cellulose I in respect to its crystalline lattice. Various chemical and thermal treatments can change the lattice of cellulose I and generate other crystalline polymorphs. In terms of industrial applications, the cellulose II polymorph is the most important. It can be obtained by a transformation in the solid state such as in the mercerization process, or from regeneration from dissolved cellulose. Because the lattice of cellulose II is thermodynamically more stable, it cannot be converted back to cellulose I. One common method to characterize cellulose polymorphs is by X-ray diffraction studies. The major peaks in the X-ray diffractograms of crystalline cellulose materials are associated with reflections from the 101, 10î and 002 planes (with increasing 2θ values) corresponding to spacings of 6.01, 5.35 and 3.94 Å in the case of cellulose I and of 7.19, 4.42 and 4.06 Å in the case of cellulose II [See reference a) by Atalla and Nagel]. Native cellulose (cellulose I) derived from wood pulps typically shows poor resolution of the 101 and 10î peaks (peaks with lower 2θ values than the 002 peak). When cellulose I is converted to cellulose II by mercerization, an increase of 101 spacing and a decrease of 10î spacing occur, which leads to a lower 2θ value for the 101 peak and a higher 2θ value for the 10î peak for the mercerized material (cellulose II).
Hydrolysis of cellulose with sulphuric acid, H2SO4, in an aqueous medium under carefully controlled conditions gives cellulose whiskers or nanocrystals with excellent reinforcing ability in polymer nanocomposites [See references by Favier et al., and by Samir et al.]. Hydrolysis of cellulose with 64% H2SO4 at 45° C. for 20 minutes-4 h, 65° C. for 15 minutes, or 70° C. for 10 minutes, introduces negatively charged sulphate groups on cellulose microcrystallites and gives the microcrystallite suspensions that form, at sufficiently high concentrations, an ordered, chiral nematic liquid crystalline phase [See references by Revol et al., and by Dong et al.]. However, the yields of the cellulose microcrystallites that are capable of forming stable colloidal suspensions are only 34.4-48.1%.
Sulphate group can also be introduced to cellulose by treatment of cellulose with sulphur trioxide, SO3 in N,N-dimethylformamide (DMF) [See reference by Schweiger]. Various methods for manufacturing sulphated cellulose materials using sulphur trioxide, SO3 or Lewis base-SO3 complex as the sulphating agent have been described in the patent literatures (See U.S. Pat. Nos. 4,064,342; 4,141,746; and 4,389,523). All these methods involve also the use of an organic reagent such as amine and/or an organic dispersant or solvent.
West and Westland have described various methods for making superabsorbent polymers including superabsorbent cellulose (See US Pat. Application Publication 20030045707 A1). One method for making the said superabsorbent cellulose comprises sulfating a cellulosic material with sulfuric acid, dissolving the sulfated cellulose in aqueous medium and precipitating the sulfated cellulose by adding a non-aqueous material (solvent) to the aqueous medium. Another method for making the said superabsorbent cellulose comprises sulfating a cellulosic material with sulfuric acid, dissolving the sulfated cellulose in aqueous medium, and regenerating the sulfated cellulose from the aqueous medium by drying off the water.
The content of sulphate groups introduced to the cellulose whiskers, nanocrystals or microcrystallites after the sulphuric acid hydrolysis of cellulose is usually assessed by sulphur elemental analysis or by conductometric titration of the materials. Sulphur (S) contents of 0.50-0.75% have been obtained on the microcrystallites obtained from 64% H2SO4 hydrolysis of Whatman No. 1 filter paper powder, while a sulphate group content of up to 393 mmol/kg which corresponds to a S content of 1.26% has been reported on the nanocrystals obtained with a yield of 23% and from the hydrolysis of Norway spruce microcrystalline cellulose (MCC) using 64.8% H2SO4 at 80° C. for 10 min [See reference by Bondeson, et al.].
Hydrolysis of bleached softwood kraft pulp or bacterial cellulose with 65% H2SO4 at 40 or 70° C. has been shown to give cellulose microcrystals or nanocrystals with the cellulose I polymorph [See references by Araki et al., and by Grunert and Winter].
Commercial production of cellulose II from cellulose I is achieved by the viscose method or the cuprammonium method [See reference by Sasaki et al.]. Laboratory preparation of cellulose II from cellulose I has been achieved by conversion of cellulose I to amorphous cellulose by grinding in a vibrating ball mill, and then by recrystallization of the amorphous cellulose by heating with H2O [See reference by Hermans and Weidinger]. Atalla and Nagel have prepared cellulose II by dissolution of cellulose powder in 85% phosphoric acid, H3PO4, and then by slow addition of the dissolved cellulose solution into H2O at room temperature [See reference a) by Atalla and Nagel]. Atalla and Nagel have also prepared cellulose II with higher crystallinity by mercerizing MCC with 23% NaOH and then by slow dilution of the mercerizing mixture with H2O and washing of the mixture at higher temperatures such as 80° C. [See reference b) by Atalla and Nagel]. Gert has reported the preparation of cellulose II by swelling of cellulose or MCC in 68% nitric acid, HNO3, and then regenerating through pressing out the acid from the swollen cellulose, diluting with H2O to ˜12% HNO3 concentrating, heating, filtering and washing [See reference by Gert]. More recently, Sasaki et al. have described the preparation of cellulose II by solubilization of MCC in sub- or super-critical H2O at 320-400° C. (25-33 MPa), and then precipitation by decanting into H2O at 20° C. [See reference by Sasaki et al.].
Prior to the present invention, however, no soluble, sulphated cellulose has been recrystallized or recovered by simple addition of a solution of soluble, sulphated cellulose to water. In addition, no sulphated cellulose II has been isolated. Furthermore, no cellulose materials have been recovered from the spent liquors of H2SO4 hydrolysis of cellulose.